1. Field of the Invention
The present invention relates to a stack having a structure capable of easily aligning a plurality of electricity generators comprising a membrane-electrode assembly (MEA) and separators, and a fuel cell system having the same.
2. Description of the Related Art
In general, a fuel cell is an electricity generating system that directly converts chemical energy into electrical energy. It achieves this through a chemical reaction between oxygen or air containing the oxygen and hydrogen contained in hydrocarbon-grouped materials such as methanol, ethanol, natural gas, etc.
Fuel cells are classified into categories including phosphate fuel cells working at a temperature of about 150° C. to 200° C., molten carbonate fuel cells working at a high temperature of about 600° C. to 700° C., solid oxide fuel cells working at a high temperature of 1,000° C. or more, and polymer electrolyte membrane fuel cells and alkali fuel cells working at a room temperature or a temperature of 100° C. or less, depending upon the type of electrolyte. Such fuel cells work on the same principle, but differ from one another in type of fuel, operating temperature, catalyst, and electrolyte used in the cell.
The recently developed polymer electrolyte membrane fuel cell (PEMFC) has an excellent output characteristic, a low operating temperature, fast starting and response characteristics compared with other fuel cells, and uses hydrogen obtained by reforming methanol, ethanol, natural gas, etc. as fuel. Accordingly, the PEMFC has a wide range of applications such as a mobile power source for vehicles, a distributed power source for the home or buildings, and a small-sized power source for electronic apparatuses.
The PEMFC requires a fuel cell main body called a stack, a fuel tank, a fuel pump supplying fuel to the stack from the fuel tank, etc. for constituting a system. Such a fuel cell further comprises a reformer, which converts the fuel to generate hydrogen gas and supplies the hydrogen gas to the stack while supplying the fuel stored in the fuel tank to the stack. The fuel stored in the fuel tank is supplied to the reformer by means of the fuel pump which then, the reformer converts the fuel and generates hydrogen gas. Next, the stack makes the hydrogen gas and oxygen electrochemically react with each other, thereby generating electric energy.
Alternatively, a fuel cell can employ a direct methanol fuel cell (DMFC) scheme which directly supplies liquid-state fuel containing hydrogen to the stack to generate electricity. The fuel cell employing the DMFC scheme does not require the reformer, unlike the PEMFC.
In the fuel cell system described above, the stack has a stacked tower structure of several or several tens electricity generators having a membrane-electrode assembly (MEA) and separators (or bipolar plates). The membrane-electrode assembly is configured such that an anode electrode (also referred to as a “fuel electrode” or “oxidation electrode”) and a cathode electrode (also referred to as an “air electrode” or “reduction electrode”) are attached to each other with an electrolyte membrane therebetween. The separator simultaneously functions as a passageway through which oxygen and hydrogen gas required for the reaction of the fuel cell are supplied and as a conductor connecting the anode electrode and the cathode electrode of each membrane-electrode assembly to each other in series. Thus, hydrogen gas is supplied to the anode electrode and oxygen is supplied to the cathode electrode through the separator. This results in an electrochemical oxidation reaction of the hydrogen gas at the anode electrode and an electrochemical reduction reaction of oxygen at the cathode electrode. Due to flow of electrons mobilized by the reactions, electricity, heat, and water are generated.
One structural defect of the conventional stack is that it is very difficult to stack a plurality of electricity generators which have the membrane-electrode assembly and separators. Specifically, accurately aligning electrode portions of the membrane-electrode assemblies and gas flow channels of the separators has a large effect on the performance of the whole stack. In addition, completely sealing the gaps between the membrane-electrode assemblies and separators to prevent fuel gas from being leaked through the gaps becomes more and more important.